It is known that catalysts of the type generally described as Ziegler catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also well known that the properties of polymer resin product obtained by polymerizing olefins in the presence of Ziegler-type catalysts vary greatly as a function of the monomers of choice, catalyst components, catalyst modifiers and a variety of other conditions which affect the catalytic polymerization process.
Important among properties of polymer resins is average molecular weight and molecular weight distribution. High molecular weights generally signify polymers having high strength properties. The melt index value ("MI") of a polymer is a measure of its average molecular weight. However two different polyethylene resins can have the same MI value and be significantly different in the distribution of the number of molecules of various molecular weights that make up the average molecular weight. Even with the same MI value, one resin might have very different quantities of high molecular weight and low molecular weight fractions and thus exhibit very different processing characteristics and properties. Thus, the molecular weight distribution ("MWD") provides important additional information about the processability and mechanical properties of a polymer. One measure of the breadth of the MWD is Melt Index Ratio ("MIR"). The MIR is defined herein as the quotient obtained by dividing the MI value of the polymer into its high load melt index ("HLMI") value. The MI and HLMI values are given by ASTM D 1238, which measures the rate of extrusion of a resin through an orifice of specific length and diameter under prescribed conditions of temperature and pressure. (A high molecular weight material is more resistant to flow than a low molecular weight material, therefore the index is inverse; a high value signifies a greater rate of extrusion, hence a lower average molecular weight.) Condition E of ASTM D 1238 uses a temperature of 190.degree. C. and a total load of 2,160 grams (about 43.25 psi). Condition F uses the same temperature but ten times the load of condition E; its results are the HLMI value. HLMI/MI, or MIR, indicates shear response, where larger values usually indicate broader molecular weight distributions for typical high density polyethylenes.
Among Ziegler-type catalysts, supported vanadium catalysts are known for a tendency to produce high density polyethylene resins having a broad MWD. By reason of the resin's broad MWD a significant portion of the resin material may exist as polyethylene polymer chains which have a low degree of polymerization, i.e. a degree of polymerization on the order of about 5 to about 16 monomer units and correspondingly a low molecular weight of about 142 to about 450. As an example, European patent publication 099 660 teaches that unless certain precautions are taken, such ethylene oligomers are formed. The presence in the polyethylene resin of a significant proportion of low molecular weight oils or n-paraffins in the C.sub.10 -C.sub.32 range poses certain disadvantages to the use of such resin for blow molding applications. The n-paraffins have relatively low boiling points, and at elevated temperatures during processing of the polyethylene resin to form useful articles, such as blow molding of household or industrial containers, in which the resin is heated to about 400.degree. F. the n-paraffins in the resin vaporize, and, upon exposure to ambient air, condense into airborne droplets, making smoke. The smoke is considered objectionable by processors and impairs acceptability of vanadium-based broad MWD polyethylene resin product. Such low molecular weight oils can have other adverse effects. For example, since such materials have a waxy nature, they can exude into mold vent holes causing plugging of the hole and thus poor operation.
One means of reducing n-paraffins and smoke in vanadium-based polyethylene resins is to narrow the MWD of the resin, thereby reducing the level of low molecular weight n-paraffinic material. This may be accomplished using the catalyst of the invention described in U.S. Pat. No. 5,262,372. However, the seemingly simple expedient of narrowing the MWD also correspondingly decreases the processability and melt strength of the resin, factors signified by lower melt index ratios. Thus, approaching elimination or reduction of the smoke generation problem by narrowing the MWD of the resin generally gives tradeoffs between acceptability of the polyethylene as a blow molding resin and the level of smoke in a vanadium based polyethylene product.